In a mobile communication system, when a burst signal is transmitted/received between many base stations disposed in a service area and a mobile station, a position and a radio transmission/reception state of which change every moment, it is an important problem how an appropriate base station is selected, how an appropriate transmission power is to be determined, how a transmission rate in adaptive modulation is determined, and how transmission of the burst signal (data) is performed at an appropriate time, for reducing a peak transmission power and increasing a system capacity. Especially, in a multimedia communication for which data amount, required performance and quality vary variously, it has become an important problem to distribute, in response to a transmission request, a necessary and sufficient resource (power resource, frequency resource, and so forth) to each communication, and, thereby, to achieve guarantee of service and increase in user capacity.
Transmission of the burst signal in a conventional mobile communication system is performed as follows: That is, burst signals provided to a transmitting station (for example, a base station) are stored in a waiting row, one by one, the burst signals in this waiting row are taken without delay, and a thus-taken burst signal is transmitted to a receiving station (for example, a mobile station) at an immediate following transmission occasion (timing of a slot, or the like). Thereby, the burst signals provided to the transmitting station one by one are transmitted to the receiving station in a manner such that the delay becomes minimum.
Further, according to the conventional mobile communication system, a transmission power value and/or a transmission rate value are determined in accordance with a state of a channel between a transmitting station and a receiving station. That is, the channel state between the transmitting station and receiving station (for example, a signal attenuation factor depending on a distance between the transmitting station and receiving station, or the like) is measured, and, based on the measured value, the transmission power value and/or transmission rate value are determined in a manner such that the receiving station can receive the signal at a predetermined reception quality (for example, a reception level). As a result of the transmission power value and/or transmission rate value of the burst signal to be transmitted from the transmitting station to the receiving station being thus determined, the receiving station can always receive the burst signal with the predetermined reception quality even when the mutual positional relationship between the transmitting station and receiving station, the attenuation characteristic of transmission wave, and so forth vary variously due to a movement of the mobile station, change in the selected base station, and so forth.
In the mobile communication system in which the burst signal is thus transmitted from the transmitting station to the receiving station at the transmission power value and/or transmission rate value determined in accordance with the state of the radio channel between the transmitting station and receiving station, in a conventional method of transmitting the burst signal in which a transmitting time is determined in a manner such that delay of the burst signal provided to the transmitting station is minimum, there is a case where, at the time at which the transmission should be performed, the state of the channel is not satisfactory (for example, the fading comes to be degraded too much, or the like), the transmission power value determined in accordance with this state becomes too large. In such a case, if the burst signal is transmitted at the thus-determined transmission power value, the peak transmission power becomes larger, and, thereby, the power consumption in the transmitting station increases. Further, due to increase of the average transmission power value caused by the increase of the peak transmission power value, the interference to communication of other mobile stations becomes larger.
Thus, in the conventional method of transmitting the burst signal, a time at which the burst signal is to be transmitted is determined merely in a manner such that the delay becomes minimum, without consideration of the state of radio channel between the transmitting station and receiving station. Thereby, the transmission power value and/or transmission rate value determined in accordance with the state of radio channel do not necessarily become appropriate ones when the burst signal is transmitted.
Further, in such a mobile communication system, distribution of information to a mobile set is performed as follows:
Periodically, path losses (or an average thereof) of channels between a mobile set and a plurality of base stations are measured, and one base station having the minimum measured value is selected. Then, data is distributed to the mobile set from the thus-selected base station wirelessly.
The period of selecting the above-mentioned base station can be set to a relatively long one (for example, several seconds), or can be set to a relatively short one (for example, several milliseconds). When this period is set to a relatively long one, data is distributed to the mobile set substantially from the base station which is nearest from the current position of the mobile set. Thereby, it is possible to perform data distribution with an averagely stable condition. When the above-mentioned period is set to a relatively short one, one base station which has the minimum instantaneous path loss is selected one by one every minute time interval. Thereby, in a mobile communication system in which transmission power control is performed in a manner such that the reception level at a mobile set is approximately fixed, it is possible to reduce the average transmission power.
When the period of selecting the base station by which transmission is performed to the mobile set is thus relatively long, information is distributed to the mobile set from the single base station for a relatively long time interval. Accordingly, the information distributing which is performed from the single base station to the mobile set for a relatively long time interval is likely to be affected by a randomly occurring instantaneous variation in fading. For example, when the path loss in the radio channel between the mobile set and the relevant base station instantaneously increases (instantaneous increase of fading), the error rate of received information in the mobile set may increase, or instantaneously the transmission power may increase so as to compensate for this path loss.
Thus, when the period of selecting the base station by which transmission is performed to the mobile set is relatively long, reception of information at the mobile set is performed not necessarily in a satisfactory condition.
Further, when the period of selecting the base station which should perform transmission to the mobile set is relatively short as mentioned above, it is necessary to perform processing such as measurement of path loss, selection of the base station based on the measured value within a relatively short period, and so forth. Accordingly, the control amount required for the information distribution increases.
Therefore, an object of the present invention is to provide a method of and a device for transmitting a burst signal by which a transmission time of the burst signal can be determined such that, when the burst signal is transmitted, the transmission power value and/or transmission rate value determined in accordance with a state of the radio channel becomes not an inappropriate one for the mobile communication system as possible.
Further, the present invention has an object to provide an information distributing method for a mobile communication system by which, while the control amount required for information distribution can be reduced as possible, reception of information by a mobile set can be performed at a state which is as good as possible, and also, an information distribution control device by which, while the control amount required for information distribution can be reduced as possible, reception of information by a mobile set can be performed at a state which is as good as possible.